Freeing up space · What's Using Space (Storage Inspector)
What does Measure again do in What's Using Space?
Short answer: It re-measures your disk from scratch, with results appearing as they arrive. You don't need it after moves made in the window; use it after big changes made elsewhere.
It walks your disk afresh and updates every figure. Results appear as they arrive rather than all at the end, and the new reading also becomes the comparison point for the next What changed.
You don't need it after a move made from this window: the Relocated panel and the main list update themselves. It's useful after you've deleted or added lots of files outside Storage Studio and want current numbers.
Measuring is paced so your own work comes first, which is why a large disk can take a little while. See What do Gone, Pinned and Reclaimed mean?